Resistance spot welding is a process used by a number of industries to join together two or more metal workpieces. The automotive industry, for example, often uses resistance spot welding to join together pre-fabricated metal workpieces during the manufacture of vehicle closure members (e.g., a door, hood, trunk lid, or lift gate) and vehicle body structures (e.g., body sides and cross-members), among others. A number of spot welds are typically formed along a peripheral edge of the metal workpieces or some other bonding region to ensure the part is structurally sound. While spot welding has typically been practiced to join together certain similarly-composed metal workpieces—such as steel-to-steel and aluminum alloy-to-aluminum alloy—the desire to incorporate lighter weight materials into a vehicle platform has generated interest in joining an aluminum alloy workpiece to a steel workpiece by resistance spot welding. The aforementioned desire to resistance spot weld dissimilar metal workpieces is not unique to the automotive industry; indeed, it extends other industries that may utilize spot welding as a joining process including the aviation, maritime, railway, and building construction industries, among others.
Resistance spot welding relies on the resistance to the flow of an electrical current through overlapping metal workpieces and across their faying interface(s) to generate heat. To carry out such a welding process, a set of opposed spot welding electrodes is clamped at aligned spots on opposite sides of the workpiece stack-up, which typically includes two or three metal workpieces arranged in lapped configuration. An electrical current is then passed through the metal workpieces from one welding electrode to the other. Resistance to the flow of this electrical current generates heat within the metal workpieces and at their faying interface(s). When the workpiece stack-up includes an aluminum alloy workpiece and an adjacent steel workpiece, the heat generated at the faying interface and within the bulk material of those dissimilar metal workpieces initiates and grows a molten aluminum alloy weld pool that extends into the aluminum alloy workpiece from the faying interface. This molten aluminum alloy weld pool wets the adjacent faying surface of the steel workpiece and, upon cessation of the current flow, solidifies into a weld joint that bonds the two workpieces together.
In practice, however, spot welding an aluminum alloy workpiece to a steel workpiece is challenging since a number of characteristics of those two metals can adversely affect the strength—most notably the strength in peel and cross-tension—of the weld joint. For one, the aluminum alloy workpiece usually contains one or more mechanically tough, electrically insulating, and self-healing refractory oxide layers on its surface. The oxide layer(s) are typically comprised of aluminum oxides, but may include other metal oxide compounds as well, including magnesium oxides when the aluminum alloy workpiece is composed of a magnesium-containing aluminum alloy. As a result of their physical properties, the refractory oxide layer(s) have a tendency to remain intact at the faying interface where they can hinder the ability of the molten aluminum alloy weld pool to wet the steel workpiece and also provide a source of near-interface defects. Efforts have been made in the past to remove the oxide layer(s) from the aluminum alloy workpiece prior to spot welding. Such removal practices can be unpractical, though, since the oxide layer(s) have the ability to regenerate in the presence of oxygen, especially with the application of heat from spot welding operations.
The aluminum alloy workpiece and the steel workpiece also possess different properties that tend to complicate the spot welding process. Specifically, steel has a relatively high melting point (˜1500° C.) and relatively high electrical and thermal resistivities, while the aluminum alloy material has a relatively low melting point (˜600° C.) and relatively low electrical and thermal resistivities. As a result of these physical differences, most of the heat is generated in the steel workpiece during current flow. This heat imbalance sets up a temperature gradient between the steel workpiece (higher temperature) and the aluminum alloy workpiece (lower temperature) that initiates rapid melting of the aluminum alloy workpiece. The combination of the temperature gradient created during current flow and the high thermal conductivity of the aluminum alloy workpiece means that, immediately after the electrical current ceases, a situation occurs where heat is not disseminated symmetrically from the weld site. Instead, heat is conducted from the hotter steel workpiece through the aluminum alloy workpiece towards the welding electrode in contact with the aluminum alloy workpiece, which creates a steep thermal gradient between the steel workpiece and that particular welding electrode.
The development of a steep thermal gradient between the steel workpiece and the welding electrode in contact with the aluminum alloy workpiece is believed to weaken the integrity of the resultant weld joint in two primary ways. First, because the steel workpiece retains heat for a longer duration than the aluminum alloy workpiece after passage of the electrical current has ceased, the molten aluminum alloy weld pool solidifies directionally, starting from the region nearest the colder welding electrode (often water cooled) associated with the aluminum alloy workpiece and propagating towards the faying interface. A solidification front of this kind tends to sweep or drive defects—such as gas porosity, shrinkage voids, and micro-cracking—towards and along the faying interface within the weld joint where oxide film residue defects are already present. Second, the sustained elevated temperature in the steel workpiece promotes the growth of brittle Fe—Al intermetallic layers at and along the faying interface. Having a dispersion of weld defects together with excessive growth of Fe—Al intermetallic layers tends to reduce the peel and cross-tension strength of the weld joint.
In light of the aforementioned challenges, previous efforts to spot weld an aluminum alloy workpiece and a steel workpiece have employed a weld schedule that specifies higher currents, longer weld times, or both (as compared to spot welding steel-to-steel), in order to try and obtain a reasonable weld bond area. Such efforts have been largely unsuccessful in a manufacturing setting and have a tendency to damage the welding electrodes. Given that previous spot welding efforts have not been particularly successful, mechanical fasteners such as self-piercing rivets and flow-drill screws have predominantly been used instead. Mechanical fasteners, however, take longer to put in place and have high consumable costs compared to spot welding. They also add weight to the vehicle body structure—weight that is avoided when joining is accomplished by way of spot welding—that offsets some of the weight savings attained through the use of aluminum alloy workpieces in the first place. Advancements in spot welding that would make the process more capable of joining aluminum alloy and steel workpieces would thus be a welcome addition to the art.